This invention relates to gravel packing open hole completions in wellbores drilled with oil-based muds (OBM's). In particular it relates to gravel packing such wells with water-based carrier fluids without first displacing the oil-based mud with an aqueous fluid. Most particularly it relates to gravel packing with the alternate path technique while using a viscoelastic fluid as the carrier fluid.
Many wells, especially in oil fields in deep-water/subsea environments, are being drilled with synthetic/oil-based muds. Because of the extremely high cost of intervention and high production rates, these wells require a reliable completion technique that prevents sand production and maximizes productivity throughout the entire life of the well. One such technique is open-hole gravel packing.
There are two principal techniques used for gravel packing open holes: (1) the alternate path technique and (2) the water packing technique. The latter uses low-viscosity fluids, such as completion brines to carry the gravel from the surface and deposit it into the annulus between a sand-control screen and the wellbore. The alternate path technique, on the other hand, utilizes viscous carrier fluids; therefore the packing mechanisms of these two techniques are significantly different. The alternate path technique allows bypassing of any bridges that may form in the annulus, caused by for example high leakoff into the formation due to filtercake erosion, or exceeding the fracturing pressure, or shale-sloughing/shale-swelling or localized formation collapse on the sand control screens.
In unconsolidated formations, sand control measures are implemented to prevent wellbore collapse. Common practice for controlling sand displacement includes placement of a gravel pack to bold formation sand in place. The gravel pack is typically deposited around a perforated liner or screen. The gravel pack filters the sand while still allowing formation fluid to flow through the gravel, the screen and a production pipe. To alleviate the difficulties raised by long or inclined intervals, gravel packing may be carried out with alternate path or “shunt” tools. Such tools include perforated shunts adapted to receive the gravel slurry as it enters the annulus around the screen. These shunts provide alternate paths that allow the gravel slurry to be delivered even if a bridge forms in the gravel before the operation is completed.
Most of the recently discovered deep-water fields contain a high fraction of shales, which are water-sensitive, and thus are (or will be) drilled with oil-based fluids. Although these wells can also be completed with an oil-based gravel packing fluid, all except a few of them to date have been gravel packed with water-based fluids. A very large fraction of them have been completed with viscous fluids using the alternate path technique. Viscoelastic surfactant (VMS) solutions have been the most widely used carrier fluid in open hole gravel packing with the alternate path technique due to their low formation and gravel pack damage characteristics, their low drawdown requirements, their capability of incorporating filtercake cleanup chemicals into the carrier fluid, and their low friction pressures.
However, because viscoelastic surfactant solutions lose their viscosity and elasticity when they are exposed to most oils, including most oil-based-muds, a large degree of uncertainty exists for their successful use as carrier fluids in cases where alternate path screens are run in hole with a wellbore full of oil-based fluids. This is because any fluid displacements that are performed prior to gravel packing do not ensure effective displacement of the oil-based mud with a water-based fluid. Any oil-based mud not displaced may fill at least part of the shunt tubes while the screens are run into the hole. The potential consequence of such an inefficient displacement is a screen-out within the shunt tubes, just when the shunt tubes are needed to bypass any annular bridges. This is because if any oil-based mud is not displaced, the VES gel would contact that oil-based mud. Because it is known that most VES gels break when they contact hydrocarbons, operators would not use VES gels as gravel carrier fluids in situations in which they would contact oil-based muds.
There are two alternatives practiced to avoid this problem. First, is displacement of the entire wellbore to water-based fluids prior to running the alternate path screens in hole. This approach has been practiced successfully in West Africa in more than a dozen wells. A big disadvantage of this approach is that it often requires running a pre-drilled liner to stabilize the wellbore, since displacements to water-based fluids can cause shale swelling and/or collapse, preventing the screens from being run to the target interval, as has been experienced in several wells in the same area. Having to run a pre-drilled liner introduces an additional trip, which is costly (rig time). The second approach is the use of polymer solutions as gravel pack carrier fluids after running alternate path screens in hole with oil-based fluids in the wellbore. Because polymer fluids maintain their viscosity when contaminated with oil-based fluids, this approach eliminates the risk of potential screen-out in the shunt tubes compared to using conventional; viscoelastic surfactant fluids. However, in the event that losses are experienced during gravel packing, polymer fluids invading the formation are damaging and well productivity suffers. Note that such losses would occur only if the filtercake lift-off or erosion occurs or if the fracturing pressure of the formation is exceeded, any of which result in activation of the shunt tubes; i.e., shunt tubes would not be needed if the filtercake remains intact and the pressure in the open hole section remains below fracturing pressure.
Thus, it would be highly desirable to have a gravel packing method using a viscoelastic surfactant carrier fluid that was insensitive to the oil-based mud system that was used to drill the reservoir, at any ratio of oil to mud, meaning that they can maintain sufficient viscosity to perform the gravel packing at least within the time frame of placing the gravel, but would break upon contact with the produced hydrocarbons, at least within the time frame of putting the well on production after gravel packing. It would also be desirable to have a method of gravel packing an open hole completion that had been drilled with an OBM with a “less insensitive” aqueous VES gravel packing carrier fluid that may be more available or less expensive. Less insensitive aqueous VES gravel packing carrier fluids are those that are sufficiently stable long enough to carry gravel in the presence of OBM's at many, but not all, OBM/VES ratios under the conditions of the treatment. The term less insensitive aqueous VES gravel packing carrier fluids is also used for those VES fluids that are less stable to higher temperatures or higher salt concentrations than are insensitive VES fluids.